It depends a lot on the actual screensize, as you are getting moiré patterns.
To avoid this, don't use "5000" but the actual screen height in pixels like this:

float k= 15;
ret = (uv.y*texsize.y*k)%2 < 1;

You can adjust the appearance by setting the value of k. It makes rather a difference whether it is a multiple of 2 or not.

Note - the pattern is actually moving because of the line "warp = 0.01000" in the milk file. Unless you explicitly overwrite these distortion parameters (e.g. rot, warp, zoop, sx,sy,dx,dy) in your code (in the per frame or per vertex section), milkdrop takes the values from the .milk file. So you'll see a distortion effect but no corresponding code, which can be a bit confusing.

I have checked with nvidia and an AMD iGPU, there is no difference. I would be surprised if there was any, with such simple code. With the mods as suggested, you'll get a definitive behaviour which does not depend on screen size neither on GPU.

With your orginal code, the appearance is very sensitive on screen size. Sometimes I get a picture similar as yours, sometimes the fine lines as you seem to expect. There is not much point in making a screenshot, it would be rather random, and I do not know how to set a height of exactly 416 pixels.